Mould lubricant

ABSTRACT

A method for preparing a mould lubricant for use in direct chill casting of reactive metals, including magnesium, magnesium alloys, aluminum, and aluminum alloys, proceeds through combining a casting lubricant and a gaseous oxidation-inhibiting agent in a vessel external from the mold under vacuum conditions. The casting lubricant is selected from a variety of casting oils and casting greases. The gaseous oxidation-inhibiting agent is selected from a variety of inert gases, including sulfur hexafluoride (SF 6 ).

FIELD OF THE INVENTION

The present invention relates to a mould lubricant, to a method fordirect chill casting of a reactive metal which utilizes the mouldlubricant, and to a reactive metal cast by the method. The expressionreactive metal includes magnesium, magnesium alloys, aluminum andaluminium alloys (for example, aluminium/lithium alloys). Throughoutthis specification, the present invention will be described in relationto magnesium but it is to be understood that the present invention isapplicable to other reactive metals.

BACKGROUND ART

Direct chill casting is a process in which molten metal is fed to awater-cooled mould and a cast metal strand is withdrawn from the mould.Direct chill casting includes horizontal direct chill casting andvertical direct chill casting. The strand withdrawn from She mouldconsists of a solid shell surrounding a liquid core which subsequentlysolidifies. The strand is sprayed with cooling water as it exits themould to both cool the strand and to extract additional heat from withinthe water-cooled mould. A casting lubricant is used in direct chillcasting to reduce friction between the solidified shell and the face ofthe mould for the deal purposes of reducing mould wear and avoidingtearing of the shell.

Casting lubricants include casting oils and casting greases. Castingoils are typically continuously fed to the interface between the face ofthe mould and the metal by an arrangement which is either integrallyformed with the mould or which is separately constructed to functionwith the mould. Casting greases may be applied manually by brushing theface of the mould prior to casting or by use of an automatic greaselubricating system such as that disclosed in International PublicationNo. WO 94/00258. casting lubricants require high thermal stability andresistance to oxidative degradation at high temperatures. Throughoutthis specification, the expression “casting lubricant” is to beunderstood to include such casting oils and greases and includes animal,vegetable, synthetic and mineral casting oils and greases such asvegetable shortening, lard used in baking, castor oil, rape seed oil,esters, paraffins and synthetic liquids.

Magnesium is a highly reactive and thermodynamically unstable elementwith molten magnesium readily oxidizing in ambient air. Three approacheshave been used to inhibit the severe oxidation process. Salt coverfluxes may be sprinkled over the molten metal; oxygen may be excludedfrom contacting the molten metal by blanketing the molten metal with aninert gas such as helium, nitrogen or argon; or a protective cover gasmay be used to blanket the molten metal. Protective cover gasestypically comprise air and/or carbon dioxide and a small amount of aninhibiting agent which reacts/interacts with the molten metal to form afilm/layer on the molten metal which protects it from oxidation. Themechanism by which inhibiting agents protect molten reactive metals isnot well understood.

U.S. Pat. No. 1,972,317 was filed in 1932 and teaches a method forinhibiting the oxidation of readily oxidizable metals (for example,magnesium) by maintaining a fluorine containing atmosphere in contactwith the surface of the metal. Lines 70-74 of the patent state, “Thepresent invention is not concerned with, nor do the claims cover, anystep in which the inhibition of detrimental attack by atmospheric gasesupon magnesium takes place within a casting mould.”

U.S. Pat. No. 3,034,186 was filed in 1956 and teaches an aluminumcasting lubricant consisting of a dispersion of solid boric acid (H₃BO₃)in a suitable oil or oily based material. The casting lubricant is saidto inhibit metal oxidation on ingot surfaces and prevent magnesiumburning in aluminum alloys with high magnesium content. The castinglubricant is believed to have never been commercially used.

U.S. Pat. No. 4,930,566 teaches a method for continuous casting of analuminium-lithium alloy through an open-ended mould which is lubricatedby a casting lubricant. Pressurized gas is directed into the mould tocontact the molten metal immediately prior to solidification to displacethe point at which tile metal contacts the mould. The gas comprises1-15% by volume oxygen with the balance being an inert and incombustiblegas such as nitrogen, argon, carbon dioxide, helium or a mixturethereof. A variation of the teaching of U.S. Pat. No. 4,930,566 relatesto the casting or magnesium in which the pressurized gas is carbondioxide containing 10% sulphur hexafluoride (SF₆). The gas and thecasting lubricant are separately introduced into the mould.

CA 2047384 reaches a method for vertical direct chill casting in which acasting lubricant is used to lubricate the mould and a protective gascontaining a high concentration of an inhibiting agent such as SF₆ isdrawn into the mould by a self-generating vacuum.

JP 2-277098 teaches a non-ferrous metal casting lubricant consisting ofa plant oil lubricant containing 0.5-3% by weight ot powdered boronnitride which is said to improve lubricating properties and reduce theamount of lubricant required as compared with plant oil lubricant alone.

SUMMARY OF THE PRESENT INVENTION

In a first aspect, the present invention provides a mould lubricantcomprising a casting lubricant containing a gaseous oxidation inhibitingagent.

Gaseous oxidation inhibiting agents include inert gases such as nitrogen(N₂) and argon (Ar) and sulfur or fluorine containing gases such assulfur dioxide (SO₂), sulfur hexafluoride (SF₆), boron trifluoride(BF₃), sulfur and fluorine containing gases as described in U.S. Pat.No. 1,972,317, and mixtures thereof.

The mould lubricant according to the first aspect of the presentinvention may be prepared by evacuating the casting lubricant andreturning it to a desired pressure by introduction of the gaseousoxidation inhibiting agent.

In a second aspect, the present invention provides a method for directchill casting of a reactive metal through a mould, the method comprisinglubricating the mould with a mould lubricant according to the firstaspect of the present invention.

The method according to the second aspect of the present invention canbe applied to the casting of reactive metals in conventional horizontalor vertical direct chill casting equipment by substituting theconventional casting lubricant with a mould lubricant according to thefirst aspect of the present invention.

The mould lubricant may be introduced to the mould from a reservoirwhere it is maintained at, above, or below atmospheric pressure.

In a third aspect, the present invention provides a reactive metal castby the method according to the second aspect of the present invention.

Although the precise mechanism is not well understood, it is believedthat surface oxidation of a reactive metal cast according to the secondaspect of the present invention is minimized by the presence of thegaseous oxidation inhibiting agent which reacts or interacts with thesurface of the molten reactive metal.

EMBODIMENTS OF THE PRESENT INVENTION WILL NOW BE DESCRIBED BY WAY OFEXAMPLE ONLY. EXAMPLE 1

A mould lubricant according to the first aspect of the present inventionwas prepared by dissolving SF₆ in canola oil.

A 500 ml sample of canola oil was placed in a clean dry vacuum flask endevacuated to 70 kPa for approximately 14 hours. The evacuated oil flaskwas agitated and returned to atmospheric pressure by slowly bubbling SF₆into the oil with an SF₆ atmosphere being maintained above the oil. Theflask was weighed before evacuation and after SF₆ addition with theweight increase being 3.78 grams. Samples of the resulting mouldlubricant and untreated canola oil were analyzed by gas chromatography.The untreated sample was found to contain approximately 6.6% by weightdissolved air and no SF₆. The mould lubricant was found to containapproximately 4.7% by weight SF₆ and approximately 5.8% by weight air.

EXAMPLE 2

A strand of magnesium was cast on a Hertwich horizontal direct chillcasting machine fitted with a standard 100 mm×100 mm squarecross-section mould using a mould lubricant prepared. according toExample 1 to lubricate the mould.

Prior to casting, the standard oil reservoir was thoroughly cleaned, newoil lines were fitted to the oil injectors and new refractory componentswere fitted to the mould. The mould lubricant was placed in the oilreservoir and SF₆ was bubbled into the mould lubricant in the oilreservoir to maintain an SF₆ blanket above the oil to minimizeliberation of SF₆ from the mould lubricant.

As is conventional, the mould lubricant was brought up to the mould fromthe oil reservoir via the oil lines and oil injectors and smeared overthe face of the mould prior to the commencement of casting. A magnesiumstrand was cast at a casting speed of 300 mm/min and the surface of thestrand ways bright. and shiny immediately after casting indicating thatthe presence of SF₆ in the mould lubricant had inhibited oxidation ofthe surface of the magnesium strand. The surface of the strand wasanalyzed by X-ray Photoelectron Spectroscopy (XPS) which indicated thepresence of a fluoride film on the surface of the strand.

Comparative Example 1

A comparative casting trial was performed which was identical in allrespects to Example 2 except that the mould lubricant was replaced byuntreated canola oil and SF₆ was not bubbled into the oil reservoir.Immediately after casting, the resulting strand of magnesium had adiscolored surface featuring blackened areas which were believed to bedue to oxidation.

What is claimed is:
 1. A process for forming a mould lubricantcomprising an oil or grease casting lubricant and a gaseousoxidation-inhibiting agent to be used in the direct chill casting of areactive metal through a cooled mould, comprising: (a) placing an oil orgrease casting lubricant in an external vessel under vacuum conditions;(b) introducing a gaseous oxidation-inhibiting agent into the externalvessel under vacuum conditions until atmospheric pressure is reached,whereby the gaseous oxidation-inhibiting agent contacts and becomesabsorbed in the casting lubricant to thereby form the mould lubricant.2. The process of claim 1, wherein the casting lubricant is an oil. 3.The process of claim 1, wherein the casting lubricant is a grease. 4.The process of claim 1, wherein the gaseous oxidation-inhibiting agentis selected from the group consisting of nitrogen, argon,sulfur-containing gas, and fluorine-containing gas.
 5. The process ofclaim 1, wherein the gaseous oxidation-inhibiting agent is sulfurhexafluoride.
 6. A method for the direct chill casting of a reactivemetal through a cooled mould, wherein molten metal is fed of a directchill casting apparatus to the cooled mould and a cast metal strand iswithdrawn therefrom, the improvement of which comprises: (a) placing anoil or grease casting lubricant in an external vessel under vacuumconditions; (b) introducing a gaseous oxidation-inhibiting agent intothe external vessel under vacuum conditions until atmospheric pressureis reached, whereby the gaseous oxidation-inhibiting agent contacts andbecomes absorbed in the casting lubricant to thereby form a mouldlubricant; and (c) transferring the mould lubricant from the externalvessel into the direct chill casting apparatus and contacting it to thesurface of the mould prior to commencement of casting.
 7. The method ofclaim 6, wherein the reactive metal is selected from the groupconsisting of magnesium, magnesium alloys, aluminum, and aluminumalloys.
 8. The method of claim 6, wherein the reactive metal is selectedfrom the group consisting of magnesium and magnesium alloys.
 9. Themethod of claim 6, wherein the reactive metal is selected from the groupconsisting of aluminum and aluminum alloys.
 10. A process for preparinga mould lubricant for use in direct chill casting of a reactive metalthrough a direct chill mould, the mould lubricant comprising a castinglubricant and a gaseous oxidation-inhibiting agent and the processcomprising combined the casting lubricant and the gaseousoxidation-inhibiting agent externally of the mould for subsequentdelivery to the mould.
 11. A process as claimed in claim 10, wherein thecasting lubricant is a casting oil and the casting oil and the gaseousoxidation-inhibiting inhibiting agent are combined by placing thecasting oil in a vessel, subjecting the vessel to a partial vacuum, andintroducing the gaseous oxidation-inhibiting agent into the partiallyevacuated vessel.
 12. A process as claimed in claim 10, wherein thecasting lubricant is a casting oil and the casting oil and the gaseousoxidation-inhibiting agent are combined by bubbling the gaseousoxidation-inhibiting agent into the casting oil.
 13. A method forlubricating a direct chill mould in the direct chill casting of areactive metal, the method comprising the steps of: (a) preparing amould lubricant comprising a casting lubricant and a gaseousoxidation-inhibiting agent by combining the casting lubricant and thegaseous oxidation-inhibiting agent externally of the mould; and (b)delivering the mould lubricant to the mould for lubrication thereof. 14.A method as claimed in claim 13 wherein the mould lubricant is deliveredto the direct chill mould from a mould lubricant reservoir locatedexternally of the mould and wherein gaseous oxidation-inhibiting agentis bubbled into the mould lubricant within the reservoir to maintain anatmosphere substantially of gaseous oxidation-inhibiting agent above themould lubricant in the reservoir.